Hollow drilling steel rods for use in extraction of minerals, tunnel drilling, etc. (hereinafter, also referred to simply as “drilling rod(s)”) generally have a stem portion, and a thread portion positioned at each end portion in an axial direction with respect to the stem portion, and has a hollow formed in the axial direction at a center portion of a horizontal section throughout the entire length.
As this kind of drilling rod, there have been a drilling rod having a male thread portion at one end portion in an axis direction with respect to a stem portion and a female thread portion at the other end portion, a drilling rod having male thread portions at both end portions in an axis direction with respect to a stem portion, and so on.
The thread portions of the drilling rods require high wear resistance, and thus threads of this kind of drilling rods are subjected to a carburizing and quenching process or a high frequency quenching process for achieving high strength.
Conventionally, as a method of manufacturing a drilling rod, there has been employed a method including performing a cutting process on an end portion of a long hollow steel bar having a length corresponding to a final drilling rod to thereby form a thread (in a case where the thread is a female thread portion, upset forging is performed on an end portion in an axial direction to increase a diameter and then threading for forming a female thread is performed), and then performing carburizing and quenching on the entire hollow steel bar or performing high frequency quenching is partially on the thread to thereby increase the strength, whereby a drilling rod having a stem portion and a thread portion is manufactured in a seamlessly continuous integral structure.
However, since the total length of the drilling rod is so long, for example, several meters, performing threading or upset forging on the long hollow steel bar is accompanied by a technical difficulty.
In addition, in order to perform heat treatment such as carburizing and quenching process or high frequency quenching process after the threading, large-sized carburizing and quenching equipment or high frequency quenching equipment is required.
In case of a drilling rod, in order to prevent the drilling rod from being bended due to the heat treatment, it is required to perform the heat treatment with the long drilling rod having a length of, for example, 4 meters to 5 meters suspended, and a large sized heat treatment furnace is required.
Further, since the drilling rod is long, there are problems also in handling such as moving of the drilling rod.
As another manufacturing method, as shown in a schematic view of FIG. 2A, there has been a method where a stem portion A and a thread portion B are separately manufactured as separate components, and they are then joined with each other by a friction welding, so as to be integrated.
In this manufacturing method, following advantages may be obtained. Firstly, it is possible to perform threading on a short material for the thread portion, and therefore the threading is easy. Further, large-sized heat treatment equipment is not required for strengthening the thread portion, unlike a case of performing the heat treatment on a long material having a seamlessly continuous integrated structure.
However, in this manufacturing method, the following unique problems occur.
In the friction welding, the stem portion and the thread portion are matched with each other in an axial direction, with a pressure being applied, and both portions are relatively rotated so as to be joined with each other by friction heat. In this case, generated heat causes a sudden rise in temperature at and around the joint portion as shown in FIG. 2B, and raises, particularly, the temperature of the joint portion, up to 1200 degrees (° C.) or more.
Next, the friction welded portion is air-cooled, such that the vicinity of the joint portion, specifically, a portion whose temperature has risen to a high temperature is cooled from a temperature which is below the quenching temperature. As a result, that portion becomes softer since the hardness of that portion is undesirably lowered (see FIG. 2C). That is, the hardness of that portion undesirably becomes lower than that of a portion which has not been influenced by the heat.
As described above, in the manufacturing method where the stem portion and the thread portion formed separately from each other are joined with each other by friction welding so as to be integrated, in order to secure characteristics (hardness) when the thread is subject to carburizing and quenching (or nitrocarburizing and quenching) so as to achieve high strength, materials containing a low amount of Cr and a high amount of Ni is applied. However, particularly in the case of these materials, during joining by friction welding, hardness around the joint portion becomes so low that the life of hollow drilling rods is shorten because a low-hardness portion becomes the origin to break.
In order to prevent this low-hardness portion from being generated to be a weak portion in terms of strength, it is effective to perform carburizing and quenching on the drilling rod throughout the entire length thereof after the joining. However, in this case, since a heat treatment is necessarily performed on the long drilling rod, large-sized heat treatment equipment is required. Therefore, the same problems as those in a case of manufacturing a drilling rod in a seamless continuous integral structure occur.
Incidentally, solutions in which a low-hardness portion is generated by friction welding so as to be a weak portion in terms of strength are disclosed in JP-T-2000-503903, JP-T-2000-513057, and JP-T-2001-502021.
Those solutions disclosed in these documents describe preventing a hardness around a joint portion from lowering greatly by friction welding, in order to achieve manufacturing it without a heat treatment such as carburizing after friction welding. In terms of this point, objects of those solutions are the same as an object of the present invention. However, those technologies disclosed in JP-T-2000-503903, JP-T-2000-513057, and JP-T-2001-502021 are different from the present invention in terms that, since carburizing or nitrocarburizing is performed on a thread portion to achieve high strength, steels disclosed therein contain a low amount of Cr (1.50% or less of Cr) and a high amount of Ni (0.5% or more of Ni).